Chuck Fry

At this writing (late 2018), MCE Technologies at store DOT mcetech DOT com has a range of 3rd-party replacement SSDs for the Mac Pro up to 2 TB. I got their 1 TB (non-NVMe) version quite some time ago to replace the original 256GB Apple SSD, and it dropped right in with no hassles. The drive shows up in System Information as a Kingston SHPM2280P2/960G. If I were to do it again today, I’d get the 2 TB NVMe version. I have also seen genuine Apple OEM SSDs on eBay. One vendor is Bloomax. MCE’s are cheaper and claim to be every bit as fast as OEM.

I had the same problem. These screws go into threaded inserts, which in turn are screwed into the heatsink. Both have threadlocker compound applied. So the threaded insert’s threadlocker gives up first, and the threaded insert unscrews from the heatsink. Remove all 4 screws, then with a pair of needle nose pliers, hemostat, or thin 7mm wrench, hold the insert steady and unscrew the screw from it.

Try macpalace DOT com for a variety of replacement parts. But be prepared to spend a bundle.